This invention also relates to a radioimmunoconjugate comprising a radionuclide that emits .alpha.-particles, a chelating agent and an antibody specific for tumor associated antigens.
Radionuclides that emit .alpha.-particles have a number of physical characteristics that make them attractive for radioimmunotherapy.
The range of an .alpha.-particle with an energy of 5 to 8 MeV is in the order of 40 to 80 .alpha.m, which limits their effectiveness to a range of several celldiameters from the decaying atom.
Within this range however, the cytotoxicity of .alpha.-particles is extraordinary. This may be contributed to the high linear energy transfer (100 keV/.mu.m) and the high electrical charge of the particles. At doses of 100 to 200 cGy .alpha.-radiation may be 5 to 100 times as toxic as .gamma.-or .beta.-radiation.
Such a radioimmunoconjugate has been disclosed in Macklis et. al. (Science vol. 240 p1024-1026, 1988) wherein a .sup.212 Bismuth isotope is used, coupled to a monoclonal antibody by the cyclic anhydrid of DTPA (Diethylenetriamine pentaacetic acid), a derivative of the well-known chelating agent DTPA.
The monoclonal antibody described is directed against a murine antigen represented as Thy 1.2, which is present on the surface of both normal and malignant murine T-cells.
Bismuth isotopes have also been described in U.S Pat. No. 4.454.106 for the purpose of radioimmunotherapy.
Another .alpha.-particles emitting isotope mentioned for use in immunotherapy is .sup.211 Astatine (Bloomer et. al. Science vol. 212 p340-341, 1981).
.sup.212 Bismuth has a physical half-life of 60.55 minutes and .sup.211 Astatine has a physical half-life of just over seven hours.
A short physical half-life like that of Bismuth requires a very fast extraction of the isotope from its source, a very fast chelation step (including the removal of adventitiously bound metal) and the extraction must be followed by immediate administration. Every hour delay between obtaining the isotope and administering it results in a dosage which is only half the intended dose.
It is therefore necessary that the source for radioactive bismuth is available in the direct vicinity of the patient. The source described for .sup.212 Bismuth is either .sup.228 Thallium, .sup.224 Radium, or .sup.212 Pb. The first two decay chains involve a .sup.220 Rn isotope, which is a noble gas and may easily leak away. Most hospitals will not be equipped to house a source with such risks. .sup.212 Pb of course has a much too short physical half-life (10 hours) to enable a producer to achieve a good distribution.
.sup.211 Astatine has a longer physical half-life than .sup.212 Bismuth. It would perhaps be possible to locate the source for this isotope outside the hospital. However, Astatine is a halogen atom which behaves very similar to Iodine, including the well known drawbacks of accumulation in certain organs and tissues. Especially because .sup.211 Astatine compounds are unstable in vivo (Int. J. Appl. Rad. lsotop.32 p. 913 1981).
Apart from these drawbacks there is also the problem of obtaining sufficient quantities of the aforementioned isotopes, because their sources are available only in microgram quantities if at all.
The present invention provides a novel radio-immunoconjugate which includes a radionuclide which overcomes the previous mentioned problems.
Another problem arising in the field of radioimmunotherapy in humans is the availability of suitable antibodies. Murine antitumor antibodies which have been suggested, will, especially after several administrations, give rise to an immunereaction by the patient. Even fragments of these antibodies will eventually lead to this response.
The solution to this problem would be the use of human monoclonal antibodies, but these are not readily available.
A good method of obtaining human monoclonal antibodies has been disclosed in European Patent Application No. 0151030. However, this method tends to produce IgM-antibodies primarily.
A problem with IgM-antibodies is that they are fairly slow in reaching the site in the body where their antigens are located (the tumor). This may take from a day up to several days.
Therefore the normally used .alpha.-emitters such as the earlier mentioned, will have decayed for the larger part when they reach their site of action.
Among a list of others, .sup.225 Actinium has been suggested as a suitable .alpha.-emitter (Monoclonal Antibodies Drug Dev.;Proceedings of the John Jacob Abel Symposium for Drug Dev.,1982, page 159-171).
The .alpha.-emitter of interest according to the authors, is 224Ra.
The paper relates to mouse IgG-monoclonals or fragments thereof, which localize faster than human IgM-antibodies, but which will lead to an immuneresponse.
Furthermore, the authors did not try any .alpha.-emitters in therapy, but only tried 125I-IgG-conjugates in a model system.
The conjugates with .sup.125 I did not lead to any improved results over unlabeled antibodies in their experiments.
Later on the authors turned from .sup.224 Ra with a half-life of approximately three days to .sup.212 Bi, which has an even shorter half-life.